Method for exposing photoconductive materials with a mercury-thallium vapor light source

ABSTRACT

EXPOSURE OF THE CONVENTIONAL DYE SENSITIZED ZINC OXIDE RESIN BINDER COPY MATERIAL, ORGANIC PHOTOCONDUCTORS AND SELENIUM IS ACCOMPLISHED WITH A RADIATION SOURCE THAT EMITS A HIGH LEVEL OF ENERGY AT 535 NANOMETERS. THE PHOTOELECTROSTATIC COPYING METHOD INVOLVES ILLUMINATING THE ORIGINAL TO BE REPRODUCED WITH A MERCURY-VAPOR TYPE LAMP WHICH HAS BEEN MODIFIED TO INCLUDE THALLIUM VAPORS THROUGH THE INTRODUCTION OF THALLIUM HALIDE INTO THE LAMP ENVELOPE. A HIGH INTENSITY GREEN LIGHT IS EMITTED WHICH PERFORMS AS A MONOCHROMATIC ENERGY SOURCE AND FINDS PARTICULAR APPLICATION WHEN REPRODUCING ORIGINALS HAVING MORE THAN ONE COLOR INDICIA THEREON SO THAT ALL COLORS ARE REPRODUCED IN ACCORDANCE WITH THEIR RELATIVE BRIGHTNESS ON THE ORIGINAL.

July 20, 1911 R L suNTo EAL 3,594,160

METHOD FOR EXPOSI NG PHOTOCONDUCTIVE MATERIALS WITH A MERCURY-THALLIUMVAPOR LIGHT SOURCE Filed June 30, 1970 4 Sheets-Sheet I.

l I y NANOMETERS RELATIVE ENERGY NANOMETERS FIGLZ INVENTOR. ROBERT L.GUNTO MERTON R. STALEY ATT'Y.

July 20, 1971 R, -r0 EI'AL 3,594,160 METHOD FOR EXPOSING PHOTOCONDUCTIVEMATERIALS WITH A MERCURY-THALLIUM VAPOR LIGHT SOURCE Filed June 30, 19704 sheets-sheet z G Tl C! U Z LU LU 2 J LL 0:

I I I I I? 30o 400 500 600 700 NANOMETERS IG.3A

T2 v 5 J (I l-l-I Z LU LU 2 '2 l NJ O! INVENTOR. NANOMETERS ROBERTL.GUNTO FIG.3B MERTON R. STALEY QIJKW;

ATT'Y.

July 20, 1971 METHOD FOR EXPOSING Filed June 30, 1970 REFL ECTANCE(PERCENT) L GUNTO PHOTOCONDUCTIVE MATERIALS WITH A MERCURY-THALLIUMVAPOR LIGHT SOURCE 4 Sheets-Sheet 3 4 1-0 I 4' 0 I 5'20 I I bo I 6 10 1NANQMETERS INVENTOR.

ROBERT L. GUNTO MERTON R. STALEY FIG.4

ATTY.

July 20, 1971' v R. L. GUNTO EI'AL 3,594,160 METHOD FOR EXPOSINGPHOTOCONDUCTIVE MATERIALS WITH A MERCURY-THALLIUM VAPOR LIGHT SOURCE 4Sheets-Sheet 4 Filed June 30, 1970 INVENTOR. ROBERT L GUN TO MERTONR.STALEY S& 1.3%

ATT'Y United States Patent US. Cl. 961 5 Claims ABSTRACT OF THEDISCLOSURE Exposure of the conventional dye sensitized zinc oxide resinbinder copy material, organic photoconductors and selenium isaccomplished with a radiation source that emits a high level of energyat 5 35 nanometers. The photoelectrostatic copying method involvesilluminating the original to be reproduced with a mercury-vapor typelamp which has been modified to include thallium vapors through theintroduction of thallium halide into the lamp envelope. A high intensitygreen light is emitted which performs as a monochromatic energy sourceand finds particular application when reproducing originals having morethan one color indicia thereon so that all colors are reproduced inaccordance with their relative brightness on the original.

This is a continuation-in-part of copending patent application Ser. No.653,090, filed July 13, 1967, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to photoelectrostaticcopying and more particularly to the method and means of making areproduction of a multi-colored original which involves the use of aspecially adapted illuminating source capable of spectrally recognizingall the colors comprising the original so that true reproduction of theimage may be cast or projected onto a photoelectrostatic member.

The photoelectrostatic copying process involves the steps ofelectrostatically charging in the dark a photoelectrostatic member, suchas a base support on which is coated a photoconductive insulating layer,exposing said charged surface to a pattern of light and shadow producedby irradiating a graphic original with a suitable illuminating source.The areas on the photoconductive layer which have been struck by theradiant energy are rendered conductive and the charges in those areasare dissipated within the photoelectrostatic member leaving a latentelectrostatic image on the surface which corresponds to the pattern oflight and shadow.

T he latent image is developed by applying electroscopic particles,usually highly colored thermoplastic resin particles, which adhere tothe portions of the latent imagebearing surface that correspond to theimage portions of the original. The powder image may then be fixed tothe base support by any of the well-known fixing techniques.

The photoelectrostatic members in general use in this art can employinorganic photoconductive insulating metallic-ions containingcrystalline compound organic photoconductors or elementalphotoconductors.

Zinc oxide in a resin binder system is typical of the inorganiccrystalline photoconductors. Organic photoconductors may be selectedfrom polymeric types which are film forming or the monomeric materialswhich are dispersed in a resin binder. Typical of the polymeric organicphotoconductive donors are: polystyrenes, polyvinylxylenes,poly-vinylnaphthalene, poly-2-vinlnaphthalene, poly- 4-vinylbiphenyl,poly-9-vinylanthracene, poly 3 vinylpyrene, poly 2 vinylquinoline andpolyacenaphthalene. Photoconductive monomeric materials may be used suchas: aromatic hydrocarbons: naphthalene, anthracene, benzanthrene,chrysene, p-diphenylbenzene, diphenyl anthracene, triphenylene,p-quaterphenyl, sexiphenyl; heterocycles such as N-alkyl carbazole,thiodiphenylamine, oxadiazoles, e.g. 2,5-bis-(p-aminophenyl) 1,3,4oxadiazole; triazoles such as 2,5-bis-(p-aminophenyl)-l,3,4-triazole;N-arylpyrazolines such as l,3,5-triphenyl-pyrazoline; hydro imidazoles,such as 1,3-diphenyl-tetrahydroimidazole; oxazole derivatives such as2,S-diphenyloxazole-2-p-dimethylamino 4,5 diphenyloxazole; thiazolederivatives such as 2-p-dialkylaminophenyl-methyl-benzothiazole.

The response of the photoconductive system as receptors of radiantenergy in this process is most important. The spectral response of thevarious photoconductors peaks sharply in the ultraviolet and nearultarviolet region of the spectrum with sensitivity extending into theblue portion of the visible region of the spectrum.

It has been found desirable to extend the spectral response of thesephotoelectrostatic members of the addition of certain organic dyes inthe case of zinc oxide; in the case of selenium, the photoconductor iscombined with certain dopants such as arsenic and tellurium, and withorganic photoconductors, they can be treated with Lewis acids such asthe fiuorenone type compounds.

A detailed description of dye sensitization of zinc oxide may be foundin US. Pat. No. 3,052,540, issued Sept. 4, 1962, to H. G. Greig.

Sensitization of the various photoconductive systems is desirable inorder to extend the spectral response of the photoelectrostatic memberto include the visible range of the spectrum. Photoelectrostatic memberssensitized in the visible range of the spectrum has permitted the use ofincandescent filament-type energy sources instead of being limited toultraviolet sources. The technique of extending the spectral sensitivityof the photoelectrostatic member and illuminating it with anincandescent filament lamp having a matching spectral energydistribution has been successful in the photocopying art because of thesimplicity and convenience of using such incandescent 'lamps. However,they have not been without certain deficiencies. The incandescentilluminating sources are inefficient since they emit a great deal ofenergy in the infrared range of the spectrum and emit only a relativelysmall proportion of electromagnetic radiation which can be utilized bythe photoelectrostatic member for the purpose of producing latent imagesthereon.

SUMMARY OF THE INVENTION It was found that a radiant energy source whichapproximates the operation of a monochromatic energy source in thevisible range of the spectrum will photocopy a wide variety of colorsappearing on the original and the copy will have shades of gray to blackimages in direct relation to the reflectivity of the colors at the wavelength of the dominant emission. The dominant emission should be in thecentral portion of the luminosity function curve of the eye, i.e.,520-590 nanometers, so that the sensitized copy sheet is illuminatedwith energy that is optimum for the human eye. In this manner theapparent brightness of the original is consistent with its actualbrightness in the usual white light. Reference to multicolored originalsincludes typewritten copy bearing a pen-ink signature, red markings on aletter, or the use of colors in an ordinary letter head.

It is the general object of this invention to provide improvedreproduction methods and means in which a multicolored graphic originalwill have all the intelligence thereon recognized and reproduced by thesystem.

It is another object of this invention to provide improved reproductionmethods and means in which a mercury vapor-type radiant energy source isemployed which emits a high concentration of energy at a wavelength inthe visible portion of the spectrum.

It is another object of this invention to provide improved reproductionmethods and means using a radiant energy source which is speciallyadapted to irradiate a multicolored graphic original from which isreflected a pattern of light and shadow and which radiant energy sourceis capable of recognizing all the intelligence on said originalevidenced by the reproduction of the .graphic subject matter onto aphotoelectrostatic member.

It is a specific object of this invention to provide an improvedreproduction method using a radiant energy source adapted to emit a highenergy level at a wave length in the visible range in conjunction with aphotoelectrostatic member, which energy source provides the advantagesof a monochromatic system.

BRIEF DESCRIPTION OF THE DRAWINGS A complete understanding of theinvention and of these and other features and advantages thereof may begained from consideration of the following detailed description taken inconjunction with the accompanying drawings wherein one embodiment of theapparatus of the invention is illustrated. It is to be expresslyunderstood, however, that the drawings are for the purpose ofillustration and description and are not intended to limit theinvention.

In the drawings:

FIG. 1 is a curve of the spectral sensitivity of a conventional dyesensitized member; 7

FIG. 2 is a spectral curve showing the emission characteristics of theprior art filament-type illuminating source;

FIGS. 3A and 3B are spectral emission curves for the illumination sourceof this invention operated at different voltage levels;

FIG. 4 is a series of curves representing the spectral reflectance ofvarious colors which commonly appear on originals;

FIG. 5 is a schematic drawing of a copy apparatus employing theradiation source of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The problem of reproducing allthe intelligence on a graphic original, particularly one that ismulticolored, has been approached through one of three possibletheoretical methods.

(1) Use of a light sensitive material which is equally responsive to allwave lengths, with a radiation source having a spectral energydistribution that is identical to the luminosity function curve of thehuman eye.

(2) Use of a radiation source which emits equal energy at all wavelengths with a light sensitive material having a spectral sensitivitycurve that is identical to the luminosity function curve of the humaneye.

(3) A combination of light sensitive material and radiation sourcewherein the product of radiant energy emitted by the source and thesensitivity value of the material at each wave length produces a curvethat corresponds to the luminosity function curve of the human eye.

The first and second methods are purely theoretical while heretoforeknown copying systems attempt to follow the third method. The method andapparatus of the instant invention provide a solution to the problem ofreproducing an original containing different colored inks through afourth method exposure.

Referring to FIGS. 1 and 2, there is shown respectively the curve forthe spectral sensitivity of a conventional electrophotographic paper andthe spectral emission curve of a conventional filament-type lamp,respectively. An example of typical illuminating source used in this art4 heretofore is a lamp sold by Sylvania, identified as their IQ lamp,No. 1500, tubular lamp 2 /2 having a 12-inch lighted length rated at1500 watts, 230 volts.

The spectral energy distribution of this lamp plotted in terms of wavelength versus relative energy produces a continuous curve, as shown inFIG. 2. It will be observed that the curve is continuous from 250nanometers to 750 nanometers. A large part of the energy is in the nearinfrared portion of the spectrum which is in the range of from 670-750nanometers. There is only a small portion of the available radiantenergy in the visible portion of the spectrum which is effective toproduce a latent electrostatic image.

The deficiencies of using an incandescent source of the type describedis best illustrated by relating it to the spectral sensitivity curve ofa typical photoelectrostatic member, as shown in FIG. 1. This curve istypical of a member which has been prepared in accordance with theaforementioned US. Pat. No. 3,052,540. It will be observed that thecurve shows sensitivity to radiation at various wave lengths includingsome sensitivity in the visible red range of from 60 0-6 30 nanometers.The illumination source (FIG. 2) shows a steadily increasing emissionfrom violet to red. The sensitivity of the system, that is, one whichuses a material having the sensitivity curve shown in FIG. 1 and aradiant energy source having a distribution curve shown in FIG. 2, maybe represented by taking the product of the two curves. Such asensitivity curve would show the system to have its greatest sensitivityat about 620 nanometers represented by the product of the lamp emissionand copy paper sensitivity at this wave length. The peak sensitivity isin the red portion of the spectrum.

It will be appreciated that while the foregoing analysis is directed toexposing photoelectrostatic members prepared using inorganicphotoconductors with an incandescent source, the same deficiencies areapplicable to members which use selenium and organic photoconductors.

In summary, it can be said that the prior methods of exposing theelectrophotographic papers have suffered because of their contincousspectral sensitivity curves and radiant energy sources that do notclosely complement the sensitivity curve of the paper. As a result, anoriginal subject having a variety of colored entries, such as cyan,green, red, violet, yellow and magenta, will not be reproduced in itsentirety. Using a system such as first described, the red, yellow andmagenta hues will not be recognized by the illuminating source, i.e.,the radiant energy is reflected by the yellow, magenta and red colorsand thereby dissipates the electrostatic charge on thephotoelectrostatic member.

The use of a source such as a mercury vapor lamp will cause the cyancolor to go unrecognized, while the colors such as yellow, red and greenwill reproduce as black images.

The effect of illuminating a colored original with the lamp of thisinvention is analogous to the effect observed in viewing various colorsunder monochromatic light in a darkened room. In the circumstance thatthe light is from a sodium vapor source, the eye would observe the coloryellow as white, white would appear white, and the colors red, green andblack would range from gray to black. It is therefore desirable toemulate this system wherein all the colors will be reproduced and theirappearance on the reproduction will be in the approximate order ofbrightness appearing on the original.

The use of a radiant energy source that has an intense emission at awave length that is within the central portion of the luminosityfunction curve of the eye (520-590 nanometers), and preferably falls atapproximately the peak position of the luminosity curve (555nanometers), would result in the optimum photocopying process using anelectrophotographic member that is responsive to radiation at thespecific wave length of the intense emission line of the source. Theradiation source is effective as a monochromatic radiation source.

It was found that a radiation source having a high intensity emission ata wave length corresponding as closely as possible to the liminosityfunction curve of the human eye, gave an increased range of colorresponse, i.e., the ability to reproduce red, magenta and yellow colors,as well as cyan, green and violet. Such an illuminating source isbasically a mercury vapor type lamp to which has been added anothermetal, such as thallium in the form of thallium iodide, to give thedesired spectral emission. The mercury thallium discharge results in aprimary or dominant emission peak at 535 nanometers and a secondaryemission of 546 nanometers. A lamp useful in the practice of thisinvention is available from the Sylvania Electroproducts Corporation,Inc., Manchester, NH.

The spectral energy distribution is represented in FIGS. 3A and 38showing a dominant emission of very high intensity at 535 nanometers.The introduction of thallium iodide into the mercury type lamp producesother emissions at 351.9 nanometers and at 322.9 nanometers. Thespectral emission lines produced by the mercury discharge appear at 405,436, 546 and 578 nanometers. The mercury emission is greatly depressedin relation to the thallium emission and in particular the thallium lineappearing at 535 nanometers. The appearance of the various mercury andthallium peaks other than the dominant peak at 535 in the spectralenergy distribution shown in FIG. 3 are less significant in thephotoelectrostatic copying method of this invention.

The illuminating source is prepared by introducing metal halides alongwith mercury as the active metals, comprising the discharge medium ofthe lamp. The mercury vapor arc envelope is made of fused silica havinga diameter of about 612 mm., and the envelope may be as large as 25 mm.Molybdenum foil-Tungsten electrode assemblies are pressed and sealedinto the ends of the tube envelope. The electrode is a Tungsten coilwrapped on a Tungsten rod with the rod extending beyond the coil. Theenvelope with the electrode assemblies in position is then evacuated andthe metal iodide, namely thallium iodide in the instant case, isintroduced into the tube in addition to mercury in an atmosphere ofargon gas under moderate pressure. A lamp, having a diameter of 6.8 mm.,when connected in a circuit powered by a suitable power supply, consumes120 watts of power per lineal inch and will generate an envelope walltemperature in the range of from 500 C. 700 C. yielding the highintensity emission spectral line at 535 nanometers. Operating under thiscondition of excitation, a spectral energy distribution trace was made(as shown in FIG. 3) using a spectroradiometer in which the band passwas 10 nanometers. The recorder employed to produce the trace wasmanufactured by the Hewlett- Packard Company, Mosely Model #135 using 8/2 x 11 paper.

The trace of the spectral energy distribution, covering the range offrom 250-750 nanometers, is presented in terms of the relative intensityof the energy at each wavelength. The height of each peak represents therelative energy value at that spectral line. Intensity of the radiationat a particular spectral line is measured in terms of the height of thepeak and then comparing it to the peak at an adjacent spectral line.Using the band pass width of 10 nanometers and the particular recorder,the relative energy values may be relied upon to yield quantitativeratio values at the critical wave length. In the instant study thethallium line at 535 nanometers was compared to the adjacent mercuryline at 546 nanometers in terms of the ratio of the heights of the twolines as they appear on a curve obtained from the spectroradiometerstudy using the 10 nanometer band pass. It will be appreciated that theoperation of the light source anywhere in the range of from 40-200 wattsper lineal inch (diameter 6.8 mm.), will produce a wall temperature inthe range of from 500 C.700 C. and hence will result in a spectralenergy 6 distribution curve such as shown in FIGS. 3A and 3B. Theenvelope wall temperature must be in this range in order to have thethallium emit the required amount of energy.

Referring to FIGS. 3A and 3B, there is shown, respectively, thedistribution curves for the radiation source of this invention operatedat watts per inch and at a lower level of 40 watts per inch. The curvesare shown in a reduced scale from the actual curves taken from therecorder so that they may be more conveniently shown in the drawings.The height of the emission at 535 nanometers is measured as well as theheight of the mercury line at 546. The ratio of the relative intensityvalues expressed in terms of the height of the 535 manometer line to themercury represents the monochromatic character of the radiation source.The greater the ratio the greater will be the monochromatic effect ofthe energy at 535 nanometers. The lower ratio values indicate that theradiation at other wavelengths such as at 400 and 436 are being emittedin sufiicient quantity to dilute the effect of the thallium radiation.

The traces in FIGS. 3A and 3B were produced on a spectroradiometer usinga 10 nanometer band pass. The spectroradiometer is adjusted to produce atrace in which the peak at 535 nanometers is at a predetermined heightof six inches on the 8 /2 x 11 coordinate paper. With thespectroradiometer adjusted in this manner the peak heights at thevarious wave lengths are proportional to one another.

FIG. 3A represents the distribution curve of the radiation source ofthis invention operated at 125 watts per inch, 6.8 mm. diameterenvelope. The ratio of the heights of the peaks recorded at 535nanometers and at 546 nanometers based on actual measurement, is 6:1. Ithas been found that unique results in the photoelectrostatic copyingprocess of this invention are observed only when the intensity of theemission at 535 nanometers is substantially greater than the intensityof the spectral line at 546 nanometers. At the higher intensity levelsof the 535 nanometer line the secondary mercury lines at 546 nanometersand 436 nanometers tend to be suppressed. The range of operabilityexpressed as the ratio of the height of the emission as 535 nanometersto the emission at 546 nanometers is from 1.75:1 to 10:1 with thepreferred range being from 4.5: l to 7: l. The ratio measured in FIG. 3Ais 6:1 for the 125 watt/lineal inch level of operation is within thepreferred range.

Referring to FIG. 3B, there is shown the trace of the distribution curvefor a tubular lamp operated at 40 watts per lineal inch. At a lowerpower level the Wall temperature is about 500 C. which is the lowertemperature limit at which the thiallium metal is activated. It will beobserved that the mercury lines at 546 and 436 show up with a greaterrelative energy level tending to become more effective. The ratio of theheight of the thallium emission to the adjacent mercury line is 2:1which is still in the range of operability.

As the ratio decreases below the 1.75:1 level, the lamp behaves as amercury source so that emission at 436 is the dominant energy. Theresult is that yellow and magenta colors will reproduce as dark imagesand the green, blue and violet hues will reproduce as white.

Referring to FIG. 4, there is presented a series of spectral reflectancecurves for various colored inks and the like over the spectral rangefrom 400-660 nanometers. The curves are identified as Y, G, C, V, M andR corresponding to the colors yellow, green, cyan, violet, magenta andred. At 535 nanometers there is drawn a line L corresponding to thedominant emission of the lamp above described.

Each of the curves intersect the dominant emission line somewhere alongits height. The point of intersection measured along the ordinaterepresents the amount of reflectance from the surface when irradiatedwith radiation at 535 nanometers. The points of intersection areidentified with the corresponding letter bearing a prime" designation.Proceeding along the line L from zero reflectance, it will be observedthat the first point is V' followed in succession by R, M, G, C and Y.The points at which the curves intersect the line L correlate to thedegree of reflectance of that particular color in the system. V and Rhave the least reflectance and will reproduce as dark gray or black. Cand G have reflectance values of 25% and 30%, respectively. Y has thegreatest degree of reflectance, about 80% and hence will appear lightgray to white in the final reproduction.

Referring now more specifically to FIG. of the drawing, therein isillustrated a copy making apparatus which embodies the presentinvention. The apparatus includes a slidably mounted assembly 16 forreceiving a graphic original 18, such as a book, from which a copy is tobe made. The assembly 16 is reciprocated into and out of a housing (notshown) to print the graphic original 18 to be scanned in order todevelop a corresponding pattern of light and shadow. The left side ofthe apparatus includes a copy sheet feeding assembly 20 which feeds aphotoelectrostatic copy sheet 22 in synchronism with the moving original18 through a station 24 to receive a uniform electrostatic charge. Thissheet is then moved past an exposing area 26 in synchronism with themovement of the original 18 so that the charged surface is selectivelydischarged in accordance with the pattern of light and shadow producedby scanning the original, thereby producing a latent electrostaticimage.

The copy sheet 22 is then fed through a developer sta tion 28 in whichthe latent image is developed into a powder image, and subsequentmovement of the copy sheet 22 carries it into a fixing station 29 wherethe image is placed in permanent form.

The assembly 20 includes a pair of drive rollers 30 secured to a shaft32 which rest on the uppermost copy sheet 22 of a stack of sheets toprovide means for feeding a single copy sheet 22 from the assembly tothe processing stations. The rollers 30 are driven by a series of belts34, 36, passing around drive pulleys or sprockets 38, 40, which aredriven by a main drive motor 42.

A pair of feed rollers 44, 46, advance the copy sheet into the chargingstation 24. Roller 46 is secured to a shaft 48 which is driven by themain drive motor 42 through the belt 50 and pulley 52. Rollers 54 and 56receive the sheet as it leaves the exposing area 26 and ad vance it intothe developing station 28 and the fusing station 29.

The exposing assembly includes a radiation source 60 which has been madeas described hereinabove to include thallium iodide. The emissioncharacteristics of the radiation source corresponds to the curve shownin FIG. 3A of the drawings. The quartz envelope of the radiation source60 is mounted in a reflector 62 to focus the radiation on anilluminating area 64 disposed in the path of movement of the original18.

The radiation reflected from the original 18 is transmitted by theoptical system including reflective surfaces 66 and 68 on either side ofa lens 70 forming an optical path between the illuminating area 64 andthe exposing area 26.

The assembly 16 is moved by a drive assembly identified generally as 72to a position in which the original is disposed adjacent theilluminating area 64 and the assembly is moved in synchronism with themovement of the copy sheet 22 past the exposing area 26 for selectivelydischarging the charged surface of the copy sheet 22.

The assembly 16 includes a transparent table 74 which is slidablymounted on rail elements 76. The drive means 72 includes a flexibleelement or connecting cable 78 secured at one end to the table.

The cable 78 passes around a pulley 80 and a pulley 82 secured to 'theshaft 84 of a return drive motor 86. The table 74 in the forward orcopying direction is driven by motor 42 connected to the cable 78through the shaft 88 8 acting through a clutch mechanism 90 whichcouples shaft 88 with shaft 84.

It should be understood that the colors or hues under consideration arenot pure colors. However, the discussion will be applicable generally.However, where highly impure colors are employed, the actual reflectancedata may vary from the data presented herein. It should be stressed thatthis data will fit most cases.

These same colors, red, yellow and magenta when exposed to theconventional radiation sources (FIG. 1) would escape recognition or verylikely appear as a light gray reproduction. This will become apparent byreferring to FIG. 4 and observing the high reflectance for these colorsin the portion of the spectrum above 600 nanometers.

Since the inorganic photoconductors, such as zinc oxide as well asselenium and organic photoconductors, such as polyvinylcarbazole andpolyvinylbenzocarbazole are all sensitive to radiation in theultraviolet portion of the spectrum, that is 376-426, early attemptswere made in this art to employ mercury vapor lamps to expose theelectrophotographic members. It was found that the colors yellow, greenand red would reproduce as dark images and the cyan appear as white.Further, the use of certain ultraviolet absorptive materials normallyemployed in paper making cause the original subject to have a lowreflectance and hence produce a copy with a darkened background.Referring again to FIG. 4, it will be seen that at 426 nanometers thecurves Y, R and G have reflectance values less than 10%, and V less than30%. The curve C at 426 nanometers would tend to repoduce as a lightcolor. The mercury vapor source would therefore not distinguish thevarious colors Y, R and G according to their relative brightness, butthey would all reproduce with the same degree of darkness of print. 'Itshould be pointed out that operating the lamp of this invention atratios below 1.75 :1 will have the effect of a mercury vapor lamp.

A significant advantage in increased speed is realized in using theradiation sources of the instant invention. The combination of spectralemission in the ultraviolet range together with the high intensityemission along the 535 nanometer spectral emission results in decreasingthe exposure time necessary to produce a latent electrostatic image onany one of selenium-type members, organic photoconductive systems andthe inorganic zinc oxide type photoconductors sensitized to have aspectral sensitivity throughout the range of 450-680 nanometers.

Sensitivity studies using photoelectrostatic members representative ofthese general classes of photoconductors were carried out comparing theconventional incandescent source with the lamps used in this invention.The test procedures called for charging the particular member up to thesaturation voltage level and measuring the sensitivity expressed involts per second when exposed to a thallium source at a given wattageinput. The test was then repeated substituting the incandescent typesource positioned the same distance from the subject and varying thewattage input until the sensitivity measured in the first run wasduplicated. In all cases the lighted length of the lamps was 1.5 inches.

In the first series of sensitivity comparison measurements, inorganicphotoconductors were employed such as zinc oxide in a resin binder whichwas dye sensitized. At a saturation voltage level of 5 volts exposure tothe thallium source at 80 watts input recorded a sensitivity of 610volts per second. The incandescent source required 400 watts of energyinput to realize the same sensitivity.

Comparisons using organic photoconductive systems such aspolyvinylbenzocarbazole sensitized with a Lewis acid established greatersensitivity using the thallium energy source against the incandescentsource. The use of sensi tizers is described in US. Pat. No. 3,037,861,issued June 5, 1962, to Helmut Hoegl. The organic photoconductor wascharged to 2000 volts and exposure to 80 watt thallium source recorded asensitivity of 96 watts per second.

9 The incandescent source required an input of 148 watts to achieve thesame sensitivity level with the organic photoconductor system.

In the third system, selenium, the thallium source at 80 watts recordeda sensitivity of 770 volts per second and the tungsten source requiredan input of 160 watts of energy to attain the same level of sensitivity.

The speed increase is significant in that an electrophotographic memberirradiated with a conventional lamp rated at 1500 watts, as hereinabovedescribed, is processed at 15 feet per minute past an exposure window,and with the lamp of this invention rated at 800 watts the processingspeed is increased to 30 feet per minute. Exposure was made on anelectrostatic copier identified as a Bruning brand Model 2000 copier.

In summation it can be said the electrophotographic copying process ofthis invention permits the reproduction of a wider range of coloredsubject matter, produces reproductions in which the image brightnessapproximates the relative brightness of the original, and finallyresults in decreasing the time necessary to expose theelectrophotographic material.

What is claimed is:

1. The process of making a reproduction of a graphic original on aphotoelectrostatic responsive material, said material being responsiveto electromagnetic radiation in the visible portion of the spectrum,comprising the steps of:

irradiating said graphic original with an electromagnetic radiationsource having a dominant thallium emission at 535 nanometers, and asecondary mercury emission at 546 nanometers, the ratio of the level ofthe dominant emission to the level of the secondary emission is in therange of 1.75:1 to 10:1, and

projecting the pattern of light and shadow produced fromirradiating saidgraphic original onto said photoelectrostatic responsive material.

2. The process described in claim 1 wherein the photoelectrostaticresponsive material is sensitized to radiation over the range of from450-680 nanometers.

3. The process described in claim 1 wherein the ratio of the dominantthallium emission to the secondary mercury emission is in the range offrom 4.5 :1 to 7:1.

10 4. The process of making a reproduction of a multicolored graphicoriginal on a photoelectrostatic responsive material, said materialbeing responsive to electromagnetic radiation in the visible portion ofthe spectrum comprising the steps of irradiating said graphic originalwith a mercury-thallium vapor lamp having a dominant energy emission at535 nanometers corresponding to the thallium energy line and a secondaryemission at 546 nanometers corresponding to a mercury energy line toproduce a pattern of light and shadow, the ratio of said dominant energyemission to said secondary energy emission being in the range of 1.75 :1to 10: 1, and

projecting said pattern onto said material, said material beingresponsive to radiation over the range of from 45 0-680 nanometers.

5. The process described in claim 1 wherein the photoelectrostaticmaterial includes a photoconductor selected from the group consisting ofzinc oxide, selenium, organic polymeric materials, organic monomericmaterials and sensitized to be responsive to electromagnetic radiationin the range of 450 680 nanometers.

References Cited UNITED STATES PATENTS 2,297,691 10/1942 Carlson 96-12,916,622 12/1959 Nieset 961X 3,196,010 7/1965 Goife et al 961 3,251,6875/1966 Fohl et al. 961 3,453,427 7/ 1969 Leiga et al. 961X OTHERREFERENCES Purves, The Focal Encyclopedia of Photography, F0- cal Press,New York (1957), 1st ed., p. 738.

GEORGE F. LESMES, Primary Examiner R. E. MARTIN, Assistant Examiner U.S.Cl. X.R.

